The present invention relates generally to a radiography device and, more particularly, to a two-step brazed x-ray target assembly for a radiography device.
The X-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Typical X-ray tubes are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. The anode is rotated by an induction motor consisting of a cylindrical rotor built into a cantilevered axle that supports the disc-shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the X-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is referenced electrically to the ground. The X-ray tube cathode provides a focused electron beam that is accelerated across the anode-to-cathode vacuum gap and produces X-rays upon impact with the anode.
In an X-ray tube device with a rotatable anode, the target has previously consisted of a disk made of a refractory metal such as tungsten, and the X-rays are generated by making the electron beam collide with this target, while the target is being rotated at high speed. Rotation of the target is achieved by driving the rotor provided on a support shaft extending from the target. Such an arrangement is typical of rotating X-ray tubes and has remained relatively unchanged in concept of operation since its induction.
However, the operating conditions for X-ray tubes have changed considerably in the last two decades. Due to continuous demands from radiologists for higher power from X-ray tubes, more and more tubes are using composite rotating anodes with tungstenrhenium as a focal spot layer, molybdenum alloy (typically TZM) as a substrate, and brazed graphite as a heat sink.
The higher power levels increase the operating temperatures of the anode which, if high enough, may result in elevated temperature plastic hoop strain deformation of the molybdenum alloy substrate. The magnitude of the strains increases as the center of the anode is approached. Large hoop strains may induce stress in the metallurgical bond between the alloy substrate and the graphite heat sink. The magnitude of this stress imposes a limit on the maximum size, rotational speed and highest allowable temperature of the alloy substrate. Should the stress exceed a threshold value, a complete debond of the graphite heat sink can result.
The metallurgical bond made between a TZM substrate and the graphite heat sink is accomplished by elevated temperature brazing, which can be as high as 1900 degrees Celsius. Prior to brazing, the TZM substrate is typically forged to a final shape that greatly enhances the strength of the material. However, during the high temperature brazing process, this strength increase may be lost due to metallurgical transformation, or recrystallization, in the TZM, which takes place near or above 1400 degrees Celsius.
It would be desirable to have an improved X-ray tube target design which would reduce the heat needed in the brazing step to attach a molybdenum alloy substrate cap to the graphite disk to overcome problems associated with prior art structures and for improving the power limits of advanced X-ray tubes.
The present invention provides an improved joining method between a molybdenum alloy substrate cap and a graphite disk used in x-ray tube targets for computed tomography applications.
Two interrelated brazing operations are used to join the molybdenum alloy cap, typically TZM, to the graphite disk. A first brazing step joins a thin molybdenum alloy sheet to the graphite disk using either a pure zirconium or pure titanium braze to form a xe2x80x9cplatedxe2x80x9d graphite subassembly. A second brazing step joins the plated subassembly to the molybdenum alloy substrate cap using a select group of highly specialized brazed alloys to form a final assembly.
These highly specialized brazed alloys are designed to have melt temperatures below the recrystallization temperature of the molybdenum alloy substrate (about 1400 degrees Celsius) and a remelt temperature after brazing, due to the diffusion of molybdenum into the braze joint, at or near 1700 degrees Celsius. High remelt is critical to fully exploit the advantage of using a molybdenum alloy substrate for rotating anode applications. By reducing the temperature that the molybdenum alloy substrate is exposed to in the brazing steps, recrystallization of the molybdenum alloy substrate is avoided, resulting in higher yield strengths for the molybdenum substrate. These higher yield strength molybdenum substrates exhibit lower rotation hoop strains at the substrate/graphite interface that reduces the possibility of tube failure by reducing the possibility of fracture in the graphite disk.
Other objects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings.